Polymer compositions, particularly diene-based elastomer compositions, may be cured or vulcanized, with sulfur or other means to form what might be referred to as a crosslinked network. Vulcanization is a chemical process for converting the polymer chains of the elastomer into more durable compositions by chemical reaction of curatives such as sulfur which modify the polymer by forming crosslinks (bridges) between individual polymer chains. Vehicular tires usually contain components comprised of sulfur cured rubber compositions.
While the mechanism of sulfur crosslinking of diene-based elastomers to form a crosslinked rubber composition may not be fully understood, both the density of the crosslinks, or crosslink density, as well as the type of crosslinks of the rubber composition, are understood to have an effect on one or more of a rubber composition's physical properties. For example, the crosslink density of the rubber composition may be varied by changing the content of vulcanization agents such as sulfur which thereby promotes variations in the crosslinked polymer's physical properties. For example, as a general rule of thumb a relatively high crosslink density for the rubber composition promotes a more elastic material.
Physical properties affected by the density and the types of crosslinks may include but are not limited to, for example, one or more of stiffness, fatigue, tear strength and hysteresis of the rubber composition. For example, see the discussion by Lake and Thomas, Proc. of the Royal Society of London, Series A, Math. and Phys. Sciences, Vol. 300, No. 1460, Page 108, (1967) as well as Lawandy and Halim, Journal of Applied Polymer Science, Vol. 96, Pages 2440 through 2445 (2005). Further, the types of crosslinks can affect physical properties of a rubber composition as discussed by M. Klüppel, G. Heinrich, Macromolecules 27, Page 3596 (1994).
However, while some understanding exists relating to how the crosslink characteristics affect various physical properties of a rubber composition, a concept of providing two or more domains of different crosslink characteristics in a rubber composition is to be evaluated.
This invention was first conceived by contemplating the physics of elastomer blends. For example, a combination of two or more different types of elastomers as blends of elastomers to obtain rubber compositions with improved properties is a common procedure. In some instances, blending two different kinds of elastomers can result in phase separation of the individual elastomers, depending somewhat upon their miscibility (or immiscibility) with each other.
Various relationships between the chemistry of elastomer blends, morphologies of elastomer blends, individual elastomeric properties of an elastomer blend and polymer blend/copolymer based blends of elastomers have been evaluated.
However, for a purpose of combining different elastomers to obtain a rubber composition with improved physical properties, an evaluation of providing varying crosslink characteristics of a rubber composition of different elastomers is to be undertaken. In particular, instead of simply providing a blend of different elastomers to provide rubber compositions of varying physical properties, an evaluation is to be undertaken for providing a blend of similar or different elastomers of varying miscibility with a combination of different crosslink characteristics, therefore a plurality of types of crosslinks. Such evaluation can be undertaken by application of, for example, an additive that already exhibits an ability to provide a variety of crosslink characteristics for elastomers.
For this invention, an evaluation of controlled crosslink distributions (CCDs) of varied crosslink densities and/or crosslink types within an elastomer composition with the ability to be crosslinked and particularly sulfur curable rubber compositions, particularly rubber compositions containing diene-based elastomer(s), by use of a combination of sulfur-containing materials and free sulfur to cure, or crosslink, the rubber composition is to be undertaken. By crosslink type, crosslinks of different crosslink length, chemistry, and connecting different binding points on the crosslinked polymer chains are to be considered.
For such evaluation, it is contemplated that a distribution of crosslink densities and/or types within a sulfur curable elastomer-containing rubber composition could be prepared with crosslinks formed by a crosslink promoting additive CPA(1) together with crosslinks formed by a sulfur curative, alternately by an organoperoxide curative, to thereby promote formation of a distributed crosslink density and type within the sulfur curable elastomer-containing rubber composition.
Also, for such evaluation, it is further contemplated that a distribution of crosslink densities and/or types within a sulfur curable elastomer-containing rubber composition could be prepared with crosslinks formed by a crosslink promoting additive CPA(2) for combining with another ingredient (e.g. elastomer substituent) in the rubber composition to form a crosslink composite between elastomer chains, together with crosslinks formed by a sulfur curative, alternately by an organoperoxide curative, to thereby promote formation of a distributed crosslink density and type within the sulfur curable elastomer-containing rubber composition.
Representative of such ingredients (which may include substituents on an elastomer), for combining with said CPA(2) are, for example:
(A) functional groups contained on the elastomer such as example, amine, siloxy, hydroxyl and carboxyl groups and
(B) coupling agents having a moiety reactive with the CPA(2) and another moiety interactive with diene-based elastomers such as, for example, coupling agent comprised of:                (1) bis (3-trialkylsilylalkyl)polysulfide having an average of from about 2 to about 4 connecting sulfur atoms in its polysulfidic bridge, and        (2) alkoxyorganomercaptosilane.        
Representative of such bis (3-trialkoxysilylalkyl)polysulfide is comprised of, for example, bis(3-triethoxysilylpropyl)polysulfide.
For illustrative purposes to depict an idealized complex crosslinked network of a plurality of differentiated crosslinks, exemplary of such CPA(2) promoted crosslinking (crosslinked elastomer) through interaction with (combining with) participating ingredients is illustrated by an idealized formula (A) in which the (—X-CPA(2)-X—) illustrates a formative plurality of crosslinks between elastomer chains and the (Sy) illustrates formative sulfur (polysulfur, for example) crosslinks between elastomer chains:

where Sy represents at least one and alternately (more usually) an average of from 2 to 8 connecting sulfur atoms, with y therefore representing a value of 1 to at least 4 and alternately (more usually) an average of from 2 to and including 4, and
where X represents:
(1) at least one functional group contained on the elastomer such as, for example, amine, siloxy, hydroxyl and carboxyl groups, or
(2) coupling agents having a moiety reactive with the CPA(2) and another moiety interactive with diene-based elastomers such as, for example, coupling agent comprised of:                (a) bis (3-trialkylsilylalkyl)polysulfide having an average of from about 2 to about 4 connecting sulfur atoms in its polylsulfidic bridge, and        (b) alkoxyorganomercaptosilane.        
For further illustrative purposes to depict an idealized complex crosslinked network of a plurality of differentiated crosslinks, exemplary of such CPA(1) promoted crosslinking (forming a crosslinked elastomer) through interaction with (combining with) elastomer chains is illustrated by an idealized formula (B) in which the (-CPA(1)-) illustrates a formative plurality of crosslinks between elastomer chains and the (Sy) illustrates formative sulfur (polysulfur, for example) crosslinks between elastomer chains:

where Sy represents at least one and alternately (more usually) an average of from 2 to 4 connecting sulfur atoms, with y therefore representing a value of 1 to at least 4 and alternately (more usually) an average of from 2 to and including 4.
It is contemplated that such CPA additive, namely said CPA(1) and CPA(2), could be added to (mixed with) the sulfur curable rubber composition during its preliminary, or non-productive, mixing in the absence of free sulfur curative, to at least partially crosslink the rubber composition following which in a separate and subsequent mixing step (productive mixing step) free sulfur curative is added (mixed). In such manner, then, the rubber composition would contain a distributed crosslink network comprised of a first crosslinked network created by the CPA additive and a second crosslink network created by the free sulfur curative.
In the description of this invention, the terms “rubber”, “elastomer” and “rubbery polymer” may be used interchangeably unless otherwise indicated. The terms “cured”, “crosslinked” and “vulcanized” may be used interchangeably unless otherwise indicated.
The term “phr” refers to parts by weight of a non rubber ingredient per 100 parts by weight of rubber in a rubber composition.
Such terms are known to those having skill in such art.